1. Field of the Invention
The present invention relates to fluororesin coating formed on a coating surface of a metal substrate and also to a vessel having the fluororesin coating on its inner face.
2. Description of Related Art
Conventionally, as vessels for domestic or restaurant use as well as for use in the food processing industry, vessels having fluororesin coating on the surface of metal substrate have often been employed, because of their usefulness in preventing adhesion of contents to be stored therein or foodstuff to be cooked therein. Such vessels include various storage vessels, frying pans, saucepans, and cooking apparatuses such as a hot cooking plate. Hence, many of the contents or foodstuff stored or cooked therein tend to stick to the vessel surface. In order to avoid this, fluororesin coating is often formed on the inner face of the vessel, which coating provides superior heat resistance, corrosion resistance as well as good mold releasing property. Typically, the operation for forming this fluororesin coating is done by the spray coating method. And, in order to obtain better mold release property, it has been a common practice to effect a drying operation at a high temperature of 400° C. or higher for a period of about 10 minutes during the drying step for finishing the coating operation. That is, during this high-temperature drying step, a portion of the surface of the fluororesin coating is melted for smoothing out, thereby to provide gloss on the surface. This gloss represents smoothness of the surface, hence can be used as an indicator of the mold releasing property of the coating.
Incidentally, it is desired that such vessels as above are of light-weight. For this reason, aluminum is often employed as the metal substrate and the fluororesin coating is formed on the surface of this metal substrate as the coating surface. In such case, if the high-temperature operation at 400° C. or higher is effected as long as 10 minutes, the heating for such long period of time may result in deformation of the metal substrate, since the melting temperature of aluminum is in the vicinity of 500° C. The temperature employed at the drying process is elevated so high because of the heat resistance of the fluororesin coating. In view of this, it is undesirable to lower the drying temperature since this will result in insufficient smoothness in the coating surface. On the other hand, if the high temperature is maintained for such long time without lowering the drying temperature as described above, this may invite disadvantageous deformation of the metal substrate. Therefore, the standard practice has been to shorten the drying period to 2 to 4 minutes with which prevention of such deformation of the metal substrate is possible. As a result, the partial melting of the fluororesin coating surface by the high temperature drying process tends to be insufficient, hence, the resultant gloss being also insufficient.
For the reasons noted above, with the conventional vessels having fluororesin coating currently available on the market, the fluororesin coating used therein does not fully provide its intended mold releasing property.
In view of the above, an object of the present invention is to provide increased gloss to the fluororesin coating so that the coating may fully provide its mold releasing property, while avoiding deformation of the metal substrate due to high temperature heating thereof.